It will depend on which factors are included in the SL Calc. The HANDWS field gets a Y or N based on the parameters in the switch. Basically, which fields are included in the Queue Time calc, and as a result the Service Level flag. For example, if you added the four items I mentions, ring,queue,delay,ctype, but the switch is only set to include ring and queue for example, you wouldn't match up. The settings are found in System Administrator under Management Settings/Calculations tab.
Also, which calls are counted as offered will play in. Which RR report are you using? If I can get to a machine that still has the canned reports on it, I'll take a look at the formula to see what it's pulling. If you were including Short calls in your formula for example (Disposition 1) you wouldn't match the RR report.